In the production of semiconductor devices, an electron beam exposure technique is a major candidate for enabling exposure of a micropattern not larger than 0.1 micrometer. An exposure device using such an electron beam exposure technique employs an electro-optical element for controlling optical characteristics of an electron beam. Electron lenses include electromagnetic ones and electrostatic ones. In particular, an electrostatic electron lens does not require a coil core, has a simpler configuration, and is more easily downsized, compared to an electromagnetic electron lens. A multi-beam system for simultaneously drawing patterns (without using any mask) by a plurality of electron beams has been proposed among electron beam exposure techniques.
An electrostatic charged particle beam lens has a structure in which electrodes are stacked with an insulator between the electrodes. When an electrical field is applied between the electrodes, a point at which a surface of the insulator, a surface of the electrodes, and a space are in contact with each other serves as a triple junction. At the triple junction, electrons are emitted from the surface of the electrode serving as a cathode due to the electrical field concentration effect. The emitted electrons directly collide with the insulator or enter the insulator after reaching an anode and being reflected from the anode to electrically charge the surface of the insulator. The electrical charge on the surface of the insulator results in the generation of an electrical field. The electrical field may deflect the trajectory of an electron beam.
To cope with such a problem, there has been proposed the idea of inhibiting electrification of the surface of an insulator by forming unevenness at the surface of the insulator (see Japanese Patent Application Laid-Open No. 2006-49702). There is also available a proposal for tapering a surface of an insulator and changing the state of an electrical field at and near a triple junction by adjusting the taper angle (see, e.g., Osamu Yamamoto et al., “Insulation performance and flashover mechanism of bridged vacuum gaps, “T. IEE Japan, Vol. 110-A, No. 12, 1980).